1. Field of the Invention
The invention pertains generally to pen recording apparatus used in making visual records of an electrical input parameter and is more particularly directed to driving mechanisms for such recorders.
2. Description of the Prior Art
In the past pen recorders have been equipped with driving mechanisms for the pen assemblies which include a coil assembly rotatably mounted within a stator assembly. The stator assembly is generally provided with a permanent magnet arrangement coupled through an air gap to a highly permeable flux path. The coil assembly rotates in the air gap due to an electrical signal applied to a driving coil. The amount of rotation or deflection is functionally related to the amplitude of the electrical signal applied. A recorder pen or stylus is physically attached to the coil assembly to record the deflection associated with the amplitude of the signal as a visual trace on a recording medium, such as chart paper or the like.
These driving mechanisms for pen recorders, normally called "penmotors" are usually designed to include a low inertia coil assembly. These coil assemblies are generally self-supporting or frameless because low coil inertia is a prerequisite for a high speed recording operation.
The frameless coil assembly further includes a through shaft on which to mount the coil for rotation. It has been found that the presence of a one-piece shaft extending lengthwise through the core magnet and mechanically attached to supporting members of the coil structure greatly enhances the structural rigidity of the coil assembly, such that it will not be deformed as a result of stresses incurred during normal operation. This is important to insure that undesired extraneous mechanical deflections will not occur to affect the accuracy and the stability of the overall drive system.
Examples of advantageous penmotors having a centrally mounted axial shaft through a core magnet or a permeable core and the coil assembly are found in U.S. Pat. No. 3,088,788 issued to A. D. Brown Jr. et al and U.S. Pat. No. 3,550,156 issued to A. R. Thompson.
However, the through shaft construction does present certain difficulties for manufacturing the coil assembly. The coil windings are usually wound first and the shaft inserted through the coil by parting the windings. Partition of the windings must be accomplished with care as the insulation on the wire can be easily damaged causing a failure of the mechanism at a future date. Such a delicate operation is time consuming and adds unnecessarily to the expense of manufacturing penmotors. It has been suggested previously that the coils be wound already partitioned but experience had shown that the coil assembly cannot readily be wound in this manner.
Another disadvantage with through shaft construction is that the axial shaft must be located through the core. This through shaft mounting requires further processing of the core to provide a centrally located aperture for the placement of the axial shaft. An aperture of this sort will degrade the performance of a core magnet as it interferes with the domain orientation and poling of the magnet. Such a degradation of the magnetic field will reduce the torque the penmotor can provide to deflect the coil assembly.
The assembly of the through shaft and a core magnet are also matters of concern. The through shaft construction necessitates that the coil assembly be assembled around the core magnet and that the delicate coil to shaft bonding and partition process be performed at this state.
This adds complexity to the already difficult manufacture. Moreover, it is relatively difficult to maintain nearly precise concentricity between the coil assembly and the core magnet since the coil assembly must have clearance around the core magnet and the through shaft must rotate with clearance in the core magnet aperture without binding.
The lack of precision tolerances stemming from the non concentricity introduced during assembly can cause other effects such as a long air gap. The shorter an air gap is made, the higher the flux density that can be maintained within the gap which can contribute significantly to the torque generated by the motor. Also, if the gap can be shortened, a more efficient heat transfer between the coil assembly and the outside shell will occur to improve performance.